Hock damage is one of the most common traumatic injuries suffered by dairy cows, but most hock injuries should be avoidable. This study investigated the effect of housing system on the development of hock damage in first lactation Holstein heifers. After calving, 60 heifers were randomly allocated to either straw yards, cubicles with butyl rubber mats or cubicles with mattresses filled with chopped tyres. The hocks of these heifers were examined in the first week after calving (week 1) and in weeks 6, 12 and 26 of lactation. Hock damage was scored as either 1 (hair loss only) or 2 (all other damage). Heifers housed in cubicles with mats had significantly worse lesions at week 26 than at week 1, while there was no change in heifers housed in cubicles with matresses, and heifers housed on straw had significantly lower lesion scores. Additionally, heifers with no lesions at calving were significantly more likely to develop lesions when kept on mats than heifers kept on straw or mattresses. These data suggest that replacing mats with mattresses in cubicles can result in a significant reduction in traumatic hock injury.